A fan rotor of the type used in an aircraft gas turbine engine includes a hub capable of rotating about a rotational axis and an array of blades extending radially from the hub. The hub includes a series of circumferentially distributed peripheral slots. Each slot extends in an axial or predominantly axial direction and has a pair of overhanging lugs, each with an inwardly facing bearing surface. When viewed in the radial direction, each slot may be linear, with the slot centerline oriented either parallel or oblique to the rotational axis, or may have a curved centerline and a corresponding curved shape. Each slot is typically open at either the forward end of the hub, the aft end of the hub, or both to facilitate installation and removal of the blades.
Each blade includes an attachment feature that occupies one of the slots and an airfoil that projects radially beyond the hub periphery. Bearing surfaces on the flanks of the attachment contact the bearing surfaces of the slot lugs to trap the blade radially in the hub. An axial retention system prevents the installed blades from migrating axially out of the slots.
During operation of the engine, the fully assembled bladed rotor rotates about its rotational axis. Each blade is followed by one of its two adjacent neighbors and is led by its other adjacent neighbor in the direction of rotation. Acordingly, each blade in the blade array is said to have a following neighbor and a leading neighbor.
During operation, a blade fragment can separate from the rest of the blade. A separation event usually results from foreign object ingestion or fatigue failure. Because the separated blade fragment can comprise a substantial portion of the entire blade, separation events are potentially hazardous and, although rare, must be safely accounted for in the design of the engine. Engine designers have devised numerous ways to safely tolerate the separation of a single blade. However it has proven inordinately difficult to accommodate the separation of two or more blades without introducing excessive weight, cost or complexity into the engine. Accordingly, it is important that the separation of one blade not provoke the separation of additional blades.
A separated blade can cause the separation of its following neighbor if the initially separated blade contacts the airfoil of the following blade. The following blade urges the initially separated blade aftwardly and, in doing so, experiences a forwardly directed reaction force. The reaction force can overwhelm the axial retention system that normally traps the following blade axially in its hub slot, thereby ejecting the blade from the slot. Accordingly, it is important that the axial retention system be able to withstand such an event.
Another desirable feature of an aircraft engine fan rotor is resistance to windmilling induced wear. Windmilling is a condition that occurs when an aircraft crew shuts down a malfunctioning or damaged engine in flight. The continued forward motion of the aircraft forces ambient air through the fan blade array causing the fan rotor to slowly rotate or “windmill”. Windmilling also occurs when wind blows through the engine of a parked aircraft. Windmilling rotational speeds are too slow to urge the blade attachment flanks centrifugally against the disk slot lugs. As a result, the blade attachments repeatedly chafe against the surfaces of the hub slots causing accelerated wear of the blade attachments and the hub. Since both the hub and blades are extremely expensive, accelerated wear is unacceptable to the engine owner.
Accelerated attachment and hub wear can be mitigated by ensuring a snug fit between the blade attachment and the hub slot. Alternatively, the attachment can be radially undersized relative to the slot with the size difference being taken up by a tightly fitting spacer that occupies the hub slot radially inboard of the blade attachment. Either way, excessive tightness complicates blade installation and removal. Moreover, surfaces that slide relative to each other during blade installation or removal are susceptible to damage from abrasive contaminants that might be present on the surfaces. Excessive tightness exacerbates the risk of damage. Accordingly, it is important not only to ensure a snug fit, but also to minimize the risk of damaging to expensive components during blade installation and removal.